Switched digital video (SDV) refers to an arrangement in which broadcast channels are only switched onto the network when they are requested by one or more subscribers, thereby allowing system operators to save bandwidth over their distribution network. In conventional cable or satellite broadcast systems, every broadcast channel is always available to all authorized subscribers. In contrast, a switched digital video channel is only available when requested by one or more authorized subscribers. Also, unlike video on-demand, which switches a singlecast interactive program to a user, switched digital video switches broadcast streams, making each stream available to one or more subscribers who simply join the broadcast stream just as they would with normal broadcast services. That is, once a switched service is streamed to a subscriber, subsequent subscribers associated with the same service group as the first subscriber can tune to the same broadcast stream. The switched digital video will often share the same resource managers and underlying resources with other on demand services.
As noted, switched digital video is largely a tool to save bandwidth. From the subscriber perspective, he or she still receives the same broadcast video service when using a switched broadcast technique; ideally the user is not able to discern that the stream was switched at all. If each one of the digital broadcast channels is being watched by subscribers in the same service group, the switched digital video approach does not yield any bandwidth savings. However, a more likely situation statistically is that only a certain number of the digital broadcast channels are being watched by subscribers in the same service group at any given time. Those channels not requested by a subscriber need not be broadcast, thereby saving bandwidth.
One way to support switched digital video is to utilize the Session Manager to manage broadcast sessions. For each channel change, the subscriber will set up a broadcast session with the Session Manager, which will determine if the requested channel is already being sent to the corresponding service group that the subscriber belongs to. The subscriber will be assigned to join the existing broadcast session if the requested channel is available at the service group or assigned to a new broadcast session if the requested channel is not available at the service group. The Session Manager will negotiate with the edge devices to allocate resources required for the session. The edge device (e.g., a digital modulator such as a QAM modulator) needs to dynamically retrieve the MPEG single program transport stream that carries the requested broadcast program (likely via IP multicast) and generate the MPEG multiple program transport stream. As part of the session setup response message, the video tuning parameters such as frequency and MPEG program number are sent back to the subscriber to access the requested broadcast channel.
When a viewer begins watching a SDV channel, the bandwidth of the QAM modulator distributing the SDV channel is reduced. That is, each time a SDV channel is bound to a QAM modulator its remaining available bandwidth decreases. Since bandwidth resources are limited, it is possible for a blocking situation to arise in which a new SDV channel cannot be bound to a QAM modulator due to lack of bandwidth. This is especially true if viewers frequently “channel surf” through “long tail” (i.e., infrequently viewed) content or turn off their television while leaving on their set top terminal so that it continues to receive a SDV channel. The increasing usage of DVRs to record programming may also adversely impact the available bandwidth if less popular programming is recorded.